This invention relates generally to liquid crystal display panels, and more particularly, to signal waveforms for driving pixels cells of a liquid crystal display panel.
A liquid crystal display (LCD) generally includes a backplate substrate, a faceplate substrate and a liquid crystal material sealed between the two. Polarizers, colorizing filters and spacers also are included between the substrates. The liquid crystal is an oily substance that flows like a liquid, but has a crystalline order in the arrangement of its molecules. An electrical field is applied to thread-like or nematic liquid crystal molecules which respond by reorienting themselves along electric field lines. Such orientation of the molecules causes light to be polarized along the particular orientation. Polarizers then transmit or block the light depending on the polarization. The backplate typically is a glass substrate on which are formed a horizontal scanning circuit, a vertical scanning circuit and a pixel region. For an active matrix LCD, the glass substrate is essentially a large integrated circuit having thousands or millions of thin-film transistor (TFT) switches. The TFT switches form horizontal and vertical scanning circuits. The TFT switches define respective cells of a pixel region. Each cell serves as a color pixel.
The TFT switches become more densely packed as resolution increases. This increases the probability of coupling between adjacent circuits. In particular, row pulses can couple to a source line, directly affecting the stored voltage level, which in turn affects a resulting gray scale tone. The degree of coupling is dependent on the signal on the drain line. Thus, the degree of gray shift is data-dependent. This results in visible crosstalk artifacts.
Because the coupling between adjacent TFT panel circuits is through a parasitic capacitor, the coupling gets worse at higher frequencies. The rectangular pulses used for the row line selects have significant harmonic content. It is the high frequency energy from the harmonics of the edges which causes the most problems. One solution has been to add components which filter the drive pulse edges to lower the harmonic content. This solution however decreases the performance of the switches at higher resolutions, and consumes additional power. Accordingly, a more effective, less power consumptive solution is needed for avoiding coupling of adjacent panel circuits on a TFT panel.
Alternative LCD displays are formed by passive matrix designs formed with `super twist nematic` (STN) or `double super twist nematic` switches. A major distinction between active matrix LCDs and passive matrix LCDs are that passive matrix LCDs do not have a transistor associated with, and located with, each pixel. A matrix of pixels is formed by electrodes arranged in horizontal rows on one plate and vertical columns on the other plate to provide at pixel at each intersection. A limitation of the STN passive matrix LCD panel is its slow response time. This limitation has become more significant as multimedia and graphics applications become more prevalent. To present mid-level colors and gray scale tones the frame rate for an STN panel must be substantially faster than the response time. As STN panel cells are designed to respond faster, the frame rate for refreshing the STN panel is to be faster. The challenge that arises, however, is that as the frame rate increases, the drive pulses of a conventional drive signal become increasingly degraded. This results in visual artifacts.
STN panels are addressed by applying orthogonal waveforms to the panel rows, while driving the panel columns with a linear combination of the row waveforms. In the simplest and most common example, the rows and columns receive rectangular pulses. Ideally, the column signals are a combination of perfectly aligned rectangular pulses. However, as the row pulses get degraded, the column waveforms do not match up as a combination of row pulses.
According to one conventional addressing scheme each row is selected once during an image frame period. In an active addressing scheme each row is selected more than once in a frame period. Because more than one pixel is addressed at the same time and because pixels share a common column line, the state of one pixel can be affected by the state of another pixel. This is referred to as "crosstalk." Because the column signals are image dependent and each column electrode is capacitively coupled to every row electrode, the state of any pixel can impact the state of every other pixel. Crosstalk due to capacitive coupling is referred to as "coupling crosstalk." Visual artifacts, such as image "ghosts" are due to such coupling crosstalk.
The panel's also exhibits RC coupling effects which appear as low pass filtering. As a result, the drive pulses have exponential edges instead of sharp, precise transitions. The time constant for the exponential edges is related to various panel elements, but generally stays constant as the panel refresh rate is increased. As the refresh rate increases, the drive pulses become narrower. With the time constant for the exponential portion staying the same, the exponential portion takes up more and more of the pulse shape as the refresh rate increases. The resulting pulses look less and less ideal, and the resulting crosstalk becomes worse and worse.
Accordingly, there is a need for a method and apparatus for driving an STN panel at increasing frame rates without degradation of the drive signal pulses. Early efforts to address this problem typically involved feedback circuits which monitored the row waveforms and forced them to be rectangular. This solution is costly and inconvenient because sense points are required at the non-driven end of the row trace lines. Such solution also increases power consumption.
Another solution has involved using row pulses which are more complex, but which do not require faster frame rates. A common characteristic of these techniques is that they select more than one row at a time. This means that the drive signal for a column line must be constructed from more than one row of information. Thus, some storage area is required. In at least one implementation, an entire frame of data must be stored on the panel, which greatly increases cost and power consumption. In another implementation two rows are selected at a time, so that the on-panel storage requirements are minimized, but the display quality improvement is less.